Interest paid in a payment

Hello, I had a tough question recently in class that I couldn't figure out how to do on my financial calculator (HP10BII)

Here's the problem: You are buying a house for $150,00 with a 10% down payment and a fixed-rate mortgage for the remainder at 8% for 20 years with MONTHLY payments. How much interest is paid on the 20th payment?

Here's what I do and where I get stuck...

150,000 (.1) = 15,000

150,00 - 15,000 = 135,000 which is my PV (Present Value)

now I have my interest at 8%/12 = 0.667 (Interest)

and for my monthly payments it is 20x12 = 240 (# of payments)

Now I know there some function to figure out how much interest is paid on the 20th payment but I don't know what I enter into the calculator to get it. So for a run-down here it what I entered into my calculator

-135,000 [PV]

0.667 [I]

240 [N]

and now I'm stuck

Any help is appreciated!